A wide variety of fibrous materials have been employed in tobacco smoke filter elements. Cellulose Acetate (“CA”) has long been considered the material of choice for this application. However, the choice of materials has been limited because of the need to balance various commercial requirements. A very important property is the filtration efficiency i.e. the ability to selectively remove or reduce undesired components from mainstream cigarette smoke stream.
To achieve appropriate filtration efficiency, materials such as carbon have been incorporated into cigarette filters. A current method for incorporating adsorbent materials in cigarette filters is the physical entrapment of adsorbent particles between CA fibers. The particle size of materials used in such prior art is generally limited in the range of 500 to about 1500 microns in diameter. In order to achieve reasonable product integrity and pressure drop, smaller particles could not be used in this design. In addition, the adsorbents were found to lose activity from exposure to triacetin, a plasticizer used as a binder for the CA fibers.
An improved and more expensive design is to put certain materials in the cavity between CA plugs in a Plug/Space/Plug (P/S/P) filter configuration to limit the exposure of adsorbent to the binder. In order to keep the pressure drop through the filter within acceptable limits, coarse granulated materials in the size of about 10 to about 60 mesh are generally used. A longer shelf life of the adsorbent was achieved, but the efficiency of the filters was limited by the relatively large particle size used. Finer size adsorbent particles with shorter internal diffusive paths and higher effective surface areas cannot be used directly in this configuration due to excessive pressure drop.
Smaller particle size adsorbent/absorbent materials generally have enhanced kinetics of reaction with gas phase components because of their shorter diffusion paths to the interior surface area of such porous materials and the interior body of such absorbent materials. It was known that employing smaller absorbent particles with shorter diffusion paths can form filters with improved kinetics and capacity for gas phase filtration applications.
It has been found that a fiber with open or semi-open micro cavities is desirable for holding the adsorbent/absorbent material in place. The term “semi-open cavities” as used herein means cavities that possess openings smaller in dimension than the internal volume of the fiber in which they are formed, and that possess the ability to entrap solid fine particles in their internal volume. The term “open cavities” means the opening is the same or bigger in dimension than the internal volume of the fiber in which they are formed.
U.S. Pat. No. 5,509,430 which is incorporated by reference in its entirety for all useful purposes including all drawings relates to polymeric bicomponent fibers and to the production of tobacco smoke filters from bicomponent fibers comprising a core of a low cost, high strength, thermoplastic polymer and a bondable sheath of a material. There is a need to develop an improved filter that has better efficiency in selectively removing or reducing undesired components from mainstream cigarette smoke stream.
U.S. Pat. No. 5,191,905 issued to Tsukamoto, which is incorporated by reference in its entirety for all useful purposes including all the drawings describes a cigarette filter. The cigarette filter has a filter chip integrally joined to the cigarette section. The filter chip is formed by combining in a bundle, at least one absorptive synthetic fiber selected from the group consisting of (1) graft polymer fibers produced from irradiated polypropylene reacted with vapor phase styrene and containing adsorptive functional groups, (2) activated carbon fibers, (3) charged electret fibers and (4) magnetic plastic fibers and then chopping the combined fibers to a predetermined length. However, Tsukamoto does not teach that the fibers (1) have micro-cavities and (2) that the carbon is loaded in the micro-cavities.
U.S. Pat. No. 5,057,368 (“'368 patent”), which is incorporated by reference in its entirety for all useful purposes including all the drawings describes shaped micro-cavity fibers that are multilobal such as trilobal or quadrilobal. Other United States patent which disclose fibers which are incorporated by reference in its entireties including the drawings are: U.S. Pat. Nos. 5,902,384; 5,744,236; 5,704,966 and 5,713,971. In addition, U.S. Pat. Nos. 5,244,614 and 4,858,629 also disclose multilobal fibers and are incorporated by reference in its entirety for all useful purposes. However, there is no disclosure in these patents that these fibers can be used in cigarette filters. U.S. Pat. Nos. 5,356,704 and 5,275,859; are all assigned on the face of the Patents to Eastman Chemical Company (“Eastman”) and these patents disclose smoke filters. All these patents are incorporated by reference in their entirety for all useful purposes.
The terms “adsorbent” or “absorbent” as used herein are defined to mean that the ability of a material to take in or soak up gas components on the surface thereof or to assimilate such components into the body thereof.